Can't get category object in some templates of WooCommerce

Question

I'm using get_category() to get the category object by its ID. Here category 39 is child of category 37. For example,

Courses (37) 
    - Programming (39)

My problem is that, if I use get_category(37 or 39) in functions.php BOTH will return null. If I use get_category(37 or 39) in single-product.php 37 (the root category) will return null. If I use this same call in add-to-cart/simple.php both will return an Object.

WooCommerce functions will be called first, and then single-product.php and then add-to-cart/simple.php templates.

What is happening?
Why does it work depending on the file?

@edit

get_term( 37, 'category' );

seems to fail as well

@edit 13/7 - The correct working solution

I managed to solve this issue before reading the answers with:
$category = get_term_by('id', $category_id, 'product_cat');

References:

no, I will update it. i meant one or another. if I use 37 or 39, the term is only accessible in some files, but is not in some others. I have no idea why — Victor Ferreira, Jul 10 '16 at 03:40
Oh. So you call it with one value and then, you call it a different time, with the other value — , Jul 10 '16 at 03:41

score 4 · Accepted Answer · edited Jun 20 '20 at 09:12

You can't use get_category() or get_term() directly with an ID everywhere. You need to use more arguments listed in here (see the example below). On the templates, I think that also it depends on the displayed products (If they have this category or subcategory).

To retrieve the desired category object you will need to do it by category slug and you will use get_category_by_slug('the_slug') instead. Then you can retrieve the ID with:
$idObj = get_category_by_slug('my_category_slug'); 
$id = $idObj->term_id;

Others useful WordPress functions:
To retrieve category name based on category ID you will need to use get_the_category_by_ID().
You can retrieve also the ID by category name and you will use get_cat_ID( 'cat_name' ).

Listing Product categories and subcategories with get_category() (example):

Here it is an example of a function based on this thread, that will list all products categories and subcategories (everywhere):

function products_cats_subcats(){
    $taxonomy     = 'product_cat';
    $orderby      = 'name';
    $show_count   = 0;      // 1 for yes, 0 for no
    $pad_counts   = 0;      // 1 for yes, 0 for no
    $hierarchical = 1;      // 1 for yes, 0 for no
    $title        = '';
    $empty        = 0;

    $args = array(
        'taxonomy'     => $taxonomy,
        'orderby'      => $orderby,
        'show_count'   => $show_count,
        'pad_counts'   => $pad_counts,
        'hierarchical' => $hierarchical,
        'title_li'     => $title,
        'hide_empty'   => $empty
    );
    $all_categories = get_categories( $args );
    echo '<ul>';
    foreach ($all_categories as $cat) {
        if($cat->category_parent == 0) {
            $category_id = $cat->term_id;
            echo '<li><a href="'. get_term_link($cat->slug, 'product_cat') .'">'. $cat->name .'</a></li>';

            $args2 = array(
                'taxonomy'     => $taxonomy,
                'child_of'     => 0,
                'parent'       => $category_id,
                'orderby'      => $orderby,
                'show_count'   => $show_count,
                'pad_counts'   => $pad_counts,
                'hierarchical' => $hierarchical,
                'title_li'     => $title,
                'hide_empty'   => $empty
            );
            $sub_cats = get_categories( $args2 );
            echo '<ol>';
            if($sub_cats) {
                foreach($sub_cats as $sub_category) {
                    echo  '<li><a href="'. get_term_link($sub_category->slug, 'product_cat') .'">' . $sub_category->name .'</a></li>';
                }
            }
            echo '</ol>';
        }
    }
    echo '</ul>';
}

To use it just put it where you want: <?php products_cats_subcats() ;?>

That will display all your categories and subcategories hierarchically ordered by Name, with the corresponding link for each category or subcategory.

Then you can also use get_term_by() to get the category name or slug:
$term = get_term_by('id', $term_id, 'product_cat', 'ARRAY_A');

$term['name']; //get the WC category name
$term['slug']; //get the WC category slug

Then now you will be able to build your own functions, to feet your needs…

Reference:

the problem with get_the_category_by_ID is that it retuns only the name of the category. The fact that we can't use this function `get_category` in some templates in the same instance of Wordpress, but we can in some others seems odd to me. I mean, I thought it should find the category in the database and this would be independent of template or anything else.. — Victor Ferreira, Jul 13 '16 at 14:43

Can't get category object in some templates of WooCommerce

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